1. Field of the Invention
The present invention relates to a steering control apparatus for controlling a wheel angle by an electric motor in a vehicle in which an axial direction (wheel angle) of a wheel is controlled to control a traveling direction of the vehicle.
2. Description of the Related Art
Conventionally, there is known a steer-by-wire (SBW) mechanism, that is, a mechanism in which a handle shaft coupled with a handle and a steering mechanism for steering a tire are mechanically separated from each other, and both are controlled by using a motor or the like independently of each other.
For example, patent document 1 discloses a steer-by-wire control apparatus for determining a steering reaction force torque on the basis of a steering state (angle of a handle, and others) and a steering angle of a tire (front wheel for steering). According to the technique disclosed in this document, only a handle angle is mainly used as the steering state which becomes input information, so that the steering reaction force torque acting on a handle shaft is determined.
Besides, there is also a method in which a road surface reaction force torque is detected, and this is multiplied by a specified torque gain to determine a steering reaction force torque.
In the steer-by-wire mechanism, as compared with a conventional steering mechanism in which a handle and a tire are coupled by a mechanical link, it becomes possible to perform wide control (meaning that the change or the like of a control characteristic and a control constant becomes easy).
In such a control, so-called lead steering, in which a vehicle reaction delay or a reaction delay of a driver is compensated by causing the response performance of a front wheel angle with respect to a steering wheel input, to implement a first order lead, is a well-known control.
Conventionally, especially in the steer-by-wire mechanism, since a front wheel angle can be controlled without interfering with a handle angle, it has been considered that there is no uncomfortable feeling for a driver in the lead steering.
In the steer-by-wire (SBW) mechanism, or in a mechanically linked steering mechanism, a driver should feel a steering reaction force torque caused by a road reaction force torque to improve steering feeling. However, in the foregoing lead steering control state, when a force proportional to the road surface reaction force torque is simply returned as the steering reaction force torque to the steering wheel, a state interfering with the driver occurs due to the variation, or the like, of the torque. To interfere with a driver means, for example, a state in which the driver, having felt a torque, adds still another correction steering action, and smooth steering is not performed.
[Patent document 1] JP-A-2000-128002
[Patent document 2] JP-A-2002-154345
[Non-patent document 1] Kramer, “Potential Function And Benefits Of Electronic Steering Assistance”, CSAT, Czech Republic, 1996